1. Field of the Invention
This invention relates to solid state image sensors, and more particularly, to active pixel sensor (APS) technology.
2. Description of Related Art
Active pixel sensors, like charge coupled devices (CCD) are solid state photosensitive devices. Typically, the devices are organized into an array of photosensitive cells, with each cell in the array corresponding to a pixel. A typical application for CCD or APS image sensing arrays is in a digital camera, although there are many other known applications.
One advantage to APS devices over CCD devices is that APS technology is more compatible with metal oxide semiconductor (MOS) technology. This allows the support electronics needed to read signals from the APS array, and to process those signals, to be constructed on the same chip and at the same time as the APS array itself. This can significantly reduce the total cost of an APS technology based imaging device.
A basic prior art APS device comprises a reversed biased photosensitive region of semiconductor material that absorbs incident electromagnetic radiation and produces hole-electron pairs. The electrons generated by the incoming light are collected and held in the photosensitive region by the action of a pin diode formed between a pinning layer at the incident surface of the device and the semiconductor material in the photosensitive region.
Incoming electromagnetic radiation first passes through the pinning layer and then into the bulk of the semiconductor material forming the photosensitive region. Holes generated when the incoming electromagnetic radiation is absorbed are collected and removed from the photosensitive region by the pinning layer and the photodiode formed between the reverse-biased photosensitive region and the substrate. The pinning layer also serves to isolate the stored electrons from the semiconductor surface, which is known to provide significantly more sites for recombination than the silicon bulk.
Electrons, however, remain trapped in the photosensitive region until a transfer device removes them. The transfer device is typically a polysilicon gate and an adjacent semiconductor region. The polysilicon gate can be triggered by the application of a potential source to allow current flow between the photosensitive region and an adjacent semiconductor region. The number of electrons trapped in the photosensitive region relates to the intensity of the absorbed electromagnetic radiation and to the duration of exposure of the APS device to the incoming radiation.
Thus the current flow which occurs when the transfer device is activated determines the brightness at the pixel corresponding to the APS device. With multiple APS devices in an array, each one corresponding to a single pixel, a multiple pixel image can be built up by scanning the APS array and activating the transfer device for each cell to determine the brightness of the image at each pixel.
One problem with prior art APS designs relates to the fact that short wavelength radiation is easily absorbed in the pinning layer where there are many holes available. The bandgap energy of the semiconductor material in this layer is such that blue photons are efficiently absorbed. The electron-hole pairs created by this photon absorption will not contribute to the detected current flow when the transfer device is activated since the electrons created by the absorption in the pinning layer will quickly recombine with one of the many holes available. Thus, one goal in the construction of an APS cell is to minimize the depth of the pinning layer to decrease the number of blue photons absorbed in that layer.
A typical method of constructing the pinning layer is with ion implantation. After the ions are implanted, heat must be applied to activate this layer. However, this heat causes diffusion of the implanted ions generating a pinning layer which is thicker than desired. A pinning layer constructed in this way, while relatively shallow as compared to other layers in MOS devices, is still thick enough to absorb an undesirably high proportion of incoming short wavelength blue light.
The bandgap energy of semiconductor material in the pinning layer is comparable to the energy of photons in blue light resulting in efficient absorption of many of these photons even in a relatively shallow pinning layer. Because the absorbed photons do not reach the underlying photosensitive region, they cannot produce the necessary hole-electron pairs in that region. Thus, the blue sensitivity of a prior art APS device is reduced.
Yet another difficulty with prior art APS devices relates to the absorption of long wavelength radiation. Long wavelength red light photons have an energy less than the bandgap energy of the silicon pinning layer. Thus, they easily penetrate the pinning layer. However, for the same reason, absorption in the semiconductor material of the photosensitive region is also relatively poor. The photosensitive region of a prior art APS device needs to be quite thick in order to ensure that a high proportion of the incoming red light is absorbed in that region and will contribute to the detected current when the transfer device is activated.
The photosensitive region of a prior art APS cell is typically constructed in an epitaxially grown semiconductor layer. The thickness of this layer must be much greater than is needed for other MOS devices elsewhere on the substrate. It would be advantageous to reduce the thickness of this layer to simplify integration of the steps needed to construct the APS cell into standard MOS processes used in creating the associated support electronics. However, if this thickness is reduced, the depth of the photosensitive region is reduced with a corresponding reduction on the red light sensitivity of the APS device.
By way of example, a conventional APS cell may have a pinning layer having a thickness of 0.1 micrometers and an epitaxially grown p- layer of 5 to 8 micrometers.
Attempts to solve these problems have involved using stepped epitaxial thicknesses, or using dual parallel gate half cell designs in which each half cell is designed for red or blue absorption. However, each of these approaches is either expensive or is difficult to integrate into standard MOS production processes.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a photosensitive device with improved blue and red light sensitivities. More specifically, it is an object of the present invention to improve blue light sensitivity by decreasing thickness of the pinning layer in an active pixel sensor and to improve red light absorption at shallower depths so that the thickness of the photosensitive region can be decreased.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.